The present invention relates generally to radar systems, and more particularly, to random noise based automotive radar systems that are particularly well adapted for use in vehicle applications such as collision avoidance and smart cruise control systems, and the like.
There are many techniques being studied for use as automotive collision-avoidance or smart cruise control devices. Types of waveform modulations presently in use include, but are not limited to, pseudo-random noise sequences (binary sequences that have many characteristics of a noise waveform but which repeat after a given time interval), frequency modulation continuous wave (FMCW or swept-frequency or chirp waveforms), and medium pulse repetition frequency (where the transmit waveform modulation is a train of pulses, and range is determined by the delay between transmission and reception of the pulse).
All of these waveforms are deterministic and periodic, in that they repeat in time. This sets the conditions for fratricide; i.e., the signal generated by another similarly-equipped vehicle enters the parent vehicle receiver and gives a false indication of the presence, range, and velocity of an object. For example, if numerous vehicles are present in a relatively high-traffic-density situation, all equipped with FMCW collision-avoidance devices that operate in the same general frequency band with similar waveform characteristics (e.g., sweep rate, sweep interval, etc.), then the probability of fratricide increases very rapidly with the number of vehicles involved.
More particularly, many adaptive cruise control systems discussed in the literature utilize frequency modulated continuous wave (FMCW) waveforms, with various algorithms to determine the range to target vehicles. The repetitive nature of these waveforms make them susceptible to fratricide from other similar radar systems operating in the vicinity. In such systems, direct radiation coming from a similar radar system in an approaching vehicle, or the reflection from a target entering a receiving antenna of a vehicle other than the one that transmitted the signal, generate erroneous detection signals. In particular, it has been determined that the number of noninterfering FMCW waveforms in a given operating bandwidth is small in comparison to the number of vehicles on a busy roadway.
Another shortcoming relating to the use of FMCW signals is that multiple FM slopes are often used to resolve range ambiguities. Therefore, it is difficult to resolve more than a small number of separate target (automobiles, trucks, etc.) returns without generating "ghost" targets. Consequently, apparent returns from ranges at which no target exists are caused by large targets at ranges greater than the ambiguous range. Since the random noise waveform never repeats in time, the system exhibits either a thumbtack ambiguity function that has no ambiguities in range or velocity (Doppler), or a ridge ambiguity function that has Doppler ambiguities but no range ambiguities. For a system having a ridge ambiguity function, the Doppler ambiguities can be set such that the first ambiguity is greater than any expected closing velocity; e.g., 200 mph. For a system having either function, there are no range ghosts that must be resolved. This means that the range and velocity of any number of targets can be resolved in a single processing dwell.
Accordingly, it is an objective of the present invention to provide for improved random noise based radar systems for use in collision avoidance, smart cruise control, and other automotive applications.